JP2020036102A - Optical modulation method, optical demodulation method, transmitter, and receiver - Google Patents

Abstract

To provide an optical modulation method, an optical demodulation method, a transmitter, and a receiver, in a new optical communication system.SOLUTION: In a new optical modulation method, a color of an optical signal is cyclically shifted by a plurality of predetermined colors # 1 to #n during a symbol period T. Then, a leading color of each symbol period Tis selected from the colors # 1 to #n on the basis of transmission data. The optical demodulation method includes the steps of: capturing an optical signal with a rolling shutter camera; identifying a pixel containing the optical signal; and obtaining received data on the basis of the color obtained in each frame in the pixel.SELECTED DRAWING: Figure 2

Description

  The present invention relates to optical communication using a camera.

  In recent years, cameras are mounted on many information terminals such as smartphones. As a method for intuitively acquiring information using a camera mounted on an information terminal, communication between an LED (light emitting diode) and a camera has been proposed (Patent Document 1).

  The LED-camera communication includes (i) a method of acquiring information from an LED blinking at a low speed at a speed equal to or less than the frame rate of the camera at one symbol per frame or less (Non-Patent Documents 1 and 2), and The method is broadly divided into a method of acquiring information under a rolling shutter phenomenon from LEDs blinking at a high speed at a rate higher than the rate (Non-patent Documents 3 to 8).

JP 2018-112169 A

Casio Computer Co., Ltd., "Visible Light Communication System Picalico", http://picalico.casio.com/ja/about/, May 2018 Searched Kensuke Kuraki, Keizo Kato, Ryuta Tanaka, "LED lighting technology that can add information to objects" FUJITSU, vol. 66, no.5, pp. 88-93, Mar. 2015 H. Park, I. Yeom, and Y. Kim, "Watch me if you can exploiting the nature of light for light-to-camera communications," Proceedings of the 2017 International Conference on Embedded Wireless Systems and Networks, pp.329- 334, Feb. 2017. Z. Yang, Z. Wang, J. Zhang, C. Huang, and Q. Zhang, "Wearables can afford: Light-weight indoor positioning with visible light," Proceedings of the 13th Annual International Conference on Mobile Systems, Applications, and Services, pp.317 {330, May. 2015. F.L. Julia, C.M.Daniel, and P.A. Josep, "A reliable asynchronous protocol for VLC communications based on the rolling shutter effect," Proceedings of IEEE Global Communications Conference (GLOBE-COM), pp.1-6, Dec. 2015. H. Du, J. Han, X. Jian, T. Jung, C. Bo, Y. Wang, and XY Li, "Martian: Message broadcast via LED lights to heterogeneous smartphones," IEEE Journal on Selected Areas in Communications, vol. .35, no.5, pp.1154-1162, May.2017. N. Rajagopal, P. Lazik, and A. Rowe, "Visual light landmarks for mobile devices," Proceedings of the 13th international symposium on Information processing in sensor networks, pp.249-260, Apr. 2014. Y. Yang, J. Hao, and J. Luo, "CeilingTalk: Lightweight indoor broadcast through LED-camera communication," IEEE Transactions on Mobile Computing, vol.16, no.12, pp.3308 {3319, Dec. 2017.

  In the former method (i), even if the area where the LED light is projected on the camera screen is only a few pixels, communication is possible if the LED color can be recognized, but slow blinking of the LED is perceived by the human eye. Flicker is a problem. In Non-Patent Document 1, communication of several tens of bps is realized by switching colors of RGB-LEDs at a period of 100 ms, and using CSK (Color Shift Keying) for giving an information amount to a difference in chromaticity coordinates. However, flickering occurs in color switching at a cycle of 100 ms, and thus the environment in which Non-Patent Document 1 can be used is limited.

  In Non-Patent Literature 2, flicker-free communication at a maximum of 10 bps is realized by CSK that controls the intensity of light emitted from each element of the RGB-LED in time series and slightly changes the color of light. However, Non-Patent Document 2 assumes that LED light is irradiated for each object and information is received from one object by using a reflected wave, and light generated by a plurality of transmitters is transmitted from a distance. No consideration has been given to receiving at the same time. In addition, transmission of 16 bits takes 2 seconds or more, and applications are limited in terms of communication speed.

  The latter method (ii) has attracted attention as a method for increasing the communication speed between the LED and the camera, but has a problem of a short communication distance. This is because the size of the area where the stripes of the LED light appear on the screen due to the rolling shutter phenomenon has a dominant effect on the communication speed and communication reliability.

  As described above, there is no long-distance LED-camera communication that enables demodulation from LED light that is reflected only on a few pixels on a camera screen while suppressing flicker.

  The present invention has been made in response to such circumstances, and one of the exemplary purposes of one embodiment is to provide a new optical communication system.

  One embodiment of the present invention relates to a light modulation method. The light modulation method generates an optical signal that cyclically transits a plurality of colors in a predetermined order, and selects a leading color of each symbol period from the plurality of colors based on transmission data.

  Another embodiment of the present invention is an optical demodulation method corresponding to the above optical modulation method. In this demodulation method, a step of capturing an optical signal with a rolling shutter camera, a step of specifying a pixel including the optical signal, and a step of acquiring received data based on a color obtained in each frame at the pixel. Performing the steps.

  It is to be noted that any combination of the above-described components and any conversion of the expression of the present invention between an apparatus, a method, a system, a recording medium, a computer program, and the like are also effective as embodiments of the present invention.

  According to the present invention, a new optical communication system can be provided.

1A and 1B are diagrams illustrating an optical communication system according to the present embodiment. FIG. 4 is a diagram illustrating a modulation method according to the embodiment. FIGS. 3A and 3B are diagrams illustrating an example of a 4-DCSK signal design. FIG. 4 is a diagram illustrating an example of data transmission by 4-DCSK. FIG. 4 is a diagram illustrating exposure timing for each line. FIG. 3 is a diagram illustrating an image obtained by capturing an optical signal with a camera. FIG. 9 is a diagram illustrating a relationship between a frame period T _fc and a symbol period T _sym . FIG. 9 is a diagram illustrating a demodulation process corresponding to a change in an irradiation position. FIG. 4 is a diagram illustrating a color boundary. FIG. 7 is a diagram illustrating a method of generating and detecting a symbol error due to camera shake. It is a figure showing the measurement result of SER. It is a figure showing an example of an AR browser.

  Hereinafter, the present invention will be described based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in each drawing are denoted by the same reference numerals, and the repeated description will be omitted as appropriate. In addition, the embodiments do not limit the invention, but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

  In this embodiment, an optical communication technology using color will be described. In the present specification, "different colors" means that at least one of hue, saturation, and lightness is different.

1. System Configuration FIGS. 1A and 1B are diagrams schematically showing an optical communication system according to the present embodiment. The optical communication system 100 includes a transmitter 200 and a receiver 300. In this optical communication, a plurality of different predetermined colors (n ≧ 2) are used. For convenience, a plurality of colors are numbered # 1 to #n to distinguish them.

  Referring to FIG. The transmitter 200 includes a light source 210 and a modulator 220. The light source 210 can generate an optical signal S1 that can be switched between a plurality of predetermined colors # 1 to #n. Although the type and configuration of the light source 210 are not limited, an LED (light emitting diode), an LD (laser diode), an organic EL (Electro Luminescence) element, or the like can be used. Alternatively, a combination of a white light source and a liquid crystal device may be used as the light source 210. As will be described in detail later, the modulator 220 controls the light source 210 according to information to be transmitted (transmission data S2).

The receiver 300 includes a camera 310 and a demodulator 320. For example, the receiver 300 is an information terminal with a moving image shooting function, such as a smartphone or a tablet terminal. The camera 310 captures an optical signal S1 output from the transmitter 200 at a predetermined frame period _Tfc . The distance between the transmitter 200 and the receiver 300 is a long distance of several meters to several tens of meters, and the optical signal S1 is radiated in a spot over several pixels of the camera 310.

  The demodulator 320 processes an image generated by the camera 310 and demodulates information (received data S3) included in the optical signal S1. Demodulator 320 can be implemented by a combination of a processor (CPU) mounted on the information terminal and a software program.

  FIG. 1A shows a pair of one transmitter 200 and one receiver 300, but in reality one receiver 300 can receive optical signals from a plurality of light sources 210. In addition, one light source 210 can transmit an optical signal to a plurality of receivers.

  The outline of the optical communication system 100 has been described above. Next, a modulation method in transmitter 200 will be described.

2. Modulation method (DCSK)
FIG. 2 is a diagram illustrating a modulation method according to the embodiment. In this specification, this modulation method is referred to as DCSK (Differential Color Shift Keying). The color of the optical signal S1 cyclically transitions among a plurality of colors # 1 to #n in a predetermined order during one symbol period T _sym . In FIG. 2, the horizontal axis represents time, and shows color transition of the optical signal S1. _Tcolor is a cycle (transition cycle) at which colors transition. The symbol period T _sym is preferably an integral multiple of the frame period T _fc of the camera 310, and in the present embodiment, T _sym = T _fc . _Tc represents a cyclic period of n colors, and _Tc = _Tcolor × n. _Tc is called a carrier cycle. As shown in FIG. 2, the leading color is variable for each symbol period T _sym and can be selected from n colors # 1 to #n.

In DCSK, the leading color _#y of each symbol period T _{sym_cur} is relatively determined based on the leading color #x of the previous symbol period T _{sym_pre} , and therefore, the leading color number of two consecutive frames (The color shift amount Δ = y−x) represents the information. In DCSK using n colors (referred to as n-DCSK), n-value information can be transmitted with one symbol. It is assumed that x = x ± n and y = y ± n hold for arbitrary color numbers x and y. The color cycling and the determination of the leading color are performed by the modulator 220 of FIG.

  FIGS. 3A and 3B are diagrams illustrating an example of a 4-DCSK signal design. In 4-DCSK, 2-bit (quaternary) information can be transmitted for each symbol. As shown in FIG. 3A, "00", "01", "10", "11" are Color shift amount Δ can be assigned to 0, 1, 2, and 3.

  FIG. 3B is an example of a signal space diagram of 4-DCSK. Here, for convenience, the four colors # 1 to # 4 are mapped to four signal points having the same distance from the origin and a phase difference of π / 2. Therefore, the color shift amount Δ can be associated with the phase difference in the signal space of FIG.

The signal point is associated with the leading color of the symbol period T _sym , and represents that the color transitions counterclockwise at the transition period T _color starting from the color of the signal point in the symbol period. Considering an analogy with digital communication, one cycle of color transitions # 1 to #n can be associated with a carrier wave (cos wave) that makes one cycle in the carrier cycle _Tc . The i-th signal point #i (1 ≦ i ≦ n) whose leading color is #i can be associated with cos (θ + 2π (i−1) / n).

FIG. 4 is a diagram illustrating an example of data transmission by 4-DCSK. FIG. 4 shows five consecutive symbol periods T _{sym_0 to} T _{sym_4} , and data to be transmitted is “01”, “01”, “10”, and “00”. The leading color of the initial symbol period T _{sym — 0} is # 1.

_Attention is paid to T _{sym — 0} . Since the transmission data is “01”, the color shift amount Δ is 1, and the first color of the next symbol period T _{sym_1} is y = x + Δ = 1 + 1 = 2.

Next, attention is paid to T _{sym — 1} . Since the transmission data is “01”, the color shift amount Δ is 1, and the leading color of the next symbol period T _{sym_2} is y = x + Δ = 2 + 1 = 3.

Next, attention is paid to T _{sym_2} . Since the transmission data is "10", the color shift amount Δ is 2, and the leading color of the next symbol period T _{sym_3} is y = x + Δ = 3 + 2 = 1.

Next, attention is paid to T _{sym — 3} . Since the transmission data is “00”, the color shift amount Δ is 0, and the leading color of the next symbol period T _{sym_4} is y = x + Δ = 1 + 0 = 1.

The above is the description of DCSK. When n = 2 ^m and the frame rate is F _R = 1 / T _fc , the communication speed in n-DCSK is m × F _R (bps). In a general smartphone camera, a frame rate of 30 fps can be commonly set, so that a communication speed of 60 bps can be achieved in the case of 4-DCSK. The number of colors n may be determined in consideration of a trade-off between the communication speed and the SER (Symbol Error Rate).

Next, advantages of DCSK will be described. In the conventional CSK, the color of the optical signal S1 is the same during one symbol period. Therefore, when one symbol time exceeds several tens ms, flicker occurs. On the other hand, according to DCSK, by setting the transition period T _color to be sufficiently short, for example, about several ms (for example, 5 ms), a change in color is not perceived by human eyes, and the occurrence of flicker can be suppressed.

3. Demodulation Method Subsequently, demodulation of a DCSK signal in the receiver 300 will be described. Return to FIG. The camera 310 is a camera of a rolling shutter system, and takes one line or several lines as one block, and captures a pixel value in units of a block at regular intervals _Tr . In the present embodiment, one block includes one line.

FIG. 5 is a diagram illustrating the exposure timing for each line. CMOS image sensor, the incident light during the exposure time T _e collected for each line, the sensor will read the amount of light received over a period of T _r immediately. The length of _Tr differs depending on the model of the receiver 300, and is constant for the same model. Since the read timing of each line cannot overlap, the exposure time of the next line starts after _Tr from the start of exposure of a certain line. In the camera 310 having the number h of lines, the frame time _TF from the start of the exposure of the first line to the end of the exposure of the h-th line is expressed by Expression (1).
T _F = (h−1) T _r + T _e (1)
On the other hand, the camera 310 can be set the frame period _{T fc,} during _T fc -T _F is present as an imaging impossible gap time _{T G.} While preparing for the next frame, the camera cannot capture images. T _e is the automatic exposure correction function of the camera varies depending on the brightness of the ambient light. In normal imaging setting, T _e is very long compared to T _r, a change in the change in brightness by the LED flashing is uniform pixel values between lines becomes extremely small. In the LED-camera communication, the exposure time is set to be smaller than _Tr as much as possible in order to clearly observe the color of the LED in each line.

FIG. 6 is a diagram illustrating an image obtained by photographing the optical signal S1 by the camera 310. For simplicity, first, consider a case where the optical signal S1 from the transmitter 200 is uniformly applied to all pixels of the camera 310. As described above, the frame period _Tfc of the camera 310 is equal to the symbol period _Tsym, and the color of the optical signal S1 changes cyclically at the transition period _Tcolor . On the other hand, in the rolling shutter system, the exposure timing differs for each line (block). Therefore, a plurality of colors # 1 to #n included in the optical signal S1 are photographed by the camera 310 as a stripe pattern. The width of the band of each color is determined according to T _color and _Tr , and the same color appears on adjacent (T _color / T _r ) lines. In the example of FIG. 6, the color changes every two lines.

  Actually, the optical signal S1 is applied to only a part of the pixels of the camera 310 in a spot-like manner. Therefore, the demodulator 320 detects a pixel where the optical signal S1 is captured. Then, the pixel value of the pixel is obtained for each frame. Then, it is determined from the obtained pixel values whether the optical signal S1 is one of the multiple colors # 1 to #n.

  Then, the demodulator 320 demodulates the received data for the same pixel based on the difference (color shift amount) between the color #p obtained in the current frame and the color #q obtained in the previous frame. I do. The above is the demodulation processing in DCSK.

Next, further advantages in DCSK communication will be described. FIG. 7 is a diagram illustrating a relationship between the frame period T _fc and the symbol period T _sym . Here again, 4-DCSK is taken as an example. The transmitter 200 and receiver 300 are asynchronous, start _{t 0} and the beginning _{t 1} of the symbol period _{T sym} frame period _{T fc} does not always coincide. Generally, the timing t _{0 at} the beginning of the frame period T _fc changes according to the activation timing of the camera 310.

(I) of FIG. 7 illustrates a case where t ₀ = t ₁ . When the optical signal S1 is applied to the point A, the color # 4 is measured at the pixel corresponding to the point A in the previous symbol period T _{sym_j} , and the color # 3 is measured in the next symbol period T _{sym_j + 1} . Therefore, the color shift amount Δ is 3, and data “11” can be demodulated.

(Ii) of FIG. 7 shows a case where t ₀ <t ₁ . In the previous symbol period T _{sym_j} , color # 1 is measured at the pixel corresponding to point A, and in the next symbol period T _{sym_j + 1} , color # 4 is measured. Accordingly, the color shift amount Δ is 3, and in this case also, the data “11” can be demodulated.

Thus, according to the DCSK communication, a signal can be demodulated without depending on the timing of the frame period _Tfc and the symbol period _Tsym .

The irradiation position of the optical signal S1 is determined according to the position and the direction of the receiver 300 and is not constant. In FIG. 7, a case is considered in which the point B is irradiated with the optical signal S1. At this time, when t ₀ = t ₁ in (i), the color # 2 is measured at the pixel corresponding to the point B in the previous symbol period T _{sym_j} , and the color # 1 is measured in the next symbol period T _{sym_j + 1.} You. Therefore, the color shift amount Δ is 3, and data “11” can be demodulated.

In the case of (ii) t ₀ <t ₁ , the color # 3 is measured at the pixel corresponding to the point B in the previous symbol period T _{sym_j} , and the color # 2 is measured in the next symbol period T _{sym_j + 1} . Accordingly, the color shift amount Δ is 3, and in this case also, the data “11” can be demodulated.

  As described above, according to the DCSK communication, there is also an advantage that data can be demodulated without depending on the irradiation position of the optical signal S1.

  Furthermore, when the receiver 300 is held by a user's hand, there is no guarantee that the optical signal S1 is always maintained at the same position during communication, and a situation where the irradiation position of the optical signal S1 differs for each frame due to camera shake may occur. sell.

  FIG. 8 is a diagram illustrating demodulation processing corresponding to a change in the irradiation position. It is assumed that the optical signal S1 is applied to the point A in the preceding frame, and the optical signal S1 is applied to the point B in the subsequent frame. At point A, color # 3 is detected in the preceding frame. At point B, color # 3 is detected in the subsequent frame.

  The demodulator 320 calculates how many color numbers shift when the same optical signal is measured at the two points A and B based on the distance between the points A and B in the scanning direction. Then, using the shift amount, the color of the other point is estimated from the actually measured value of the color of one point. In the example of FIG. 8, the shift amount is 3.

  For example, the demodulator 320 estimates the color number # 2 that would have been measured at the point B in the preceding frame by adding the shift amount 3 to the color number # 3 of the point A in the preceding frame. Then, a difference between the estimated value # 2 of the color number of the point B in the preceding frame and the actually measured value # 3 of the color number of the point B in the subsequent frame is calculated, and the color shift amount Δ = 1 can be obtained.

  Alternatively, the demodulator 320 may estimate the color number # 4 that would have been measured at the point A in the subsequent frame by subtracting the shift amount 3 from the color number # 3 of the point B in the subsequent frame. . Then, the difference between the estimated value # 4 of the color number of the point A in the subsequent frame and the actually measured value # 3 of the color number of the point A in the preceding frame may be calculated to obtain the color shift amount Δ = 1.

4. Following timing design, 1 and line exposure time T _e to set the receiver side, the transition period T _color conditions in the symbol to be set at the transmitter side will be described. For clarity of color boundaries caught on the screen, it is necessary to set the T _e shorter than T _r. Under the rolling shutter system, to initiate the exposure of the next line every predetermined interval T _r, T _e> exposure time lines adjacent to each other when a T _r overlap, the color boundary is unclear . On the other hand, if _Te is excessively short, the amount of light received by the CMOS image sensor decreases, so that the color of the LED light does not appear on the screen. When one color is imaged over the entire time of exposing one line, at least the receiver needs to be able to recognize the color.

When this minimum exposure time T _{e_min,} by setting the T _e in a range satisfying the equation (2) can be immobilized and the boundary of the color caught on the screen to appear clearly.
_{T e_min ≦ T e ≦ T r} ... (2)
However, T _{e_min} is to vary factors such as power consumption, ambient light, the communication distance of the transmitter side, may T _e is set as close as T _r as possible. In some models it can not be set programmatically T _e, that the time exposed by the automatic exposure correction function is to secure the exposure time at the timing of its shorter possible.

Incidentally, it is not always necessary to satisfy the equation (2), when a T _r ≦ T _e, because the fog is generated in the exposure time of the adjacent lines of the screen, but the difference in color between adjacent lines is ambiguous, this case But demodulation is possible.

In order to suppress the flicker, T _color needs to be set shorter than the frame period of the camera. On the other hand, if _Tcolor is excessively short, even if the optical signal S1 appears on the screen, the color is switched while one line is exposed, so that the color cannot be identified. When the value of the maximum Tcolor capable of suppressing flicker and _{T flicker,} by setting the _{T color} within a range satisfying the equation (3), the color discernible appears at least one line.
_{T e + T r ≦ 2T r} ≦ T color ≦ T flicker ... (3)
The SER (Symbol Error Rate) decreases as T _color is increased within a range satisfying Expression (3). However, it should be noted that T _flicker varies depending on the number and type of colors used.

5. Color Boundaries When camera shake occurs during communication and the position of the optical signal S1 in the screen fluctuates, it is necessary to estimate the time at which the imaging time interval of the optical signal S1 between frames deviates from the frame period. . By ascertaining the boundary line at which the color of the symbol is switched in the screen, it is possible to accurately estimate the deviation. For example, since T _sym = T _fc is set, if the boundary line at which the color is switched can be imaged even once, all the boundary lines Y in the screen can be calculated. By calculating Y, the exact timing of color switching can be grasped, so that accurate estimation of the shifted time is possible. It is also possible to demodulate while predicting a plurality of boundary lines in the screen by the arithmetic processing.

FIG. 9 is a diagram illustrating a color boundary. FIG. 9 shows a case where a color boundary is imaged on the (i-1) th line and the i-th line. The ith line, not the (i-1) th line, is defined as a Y element. Although the boundary line is an integer value, the time parameter such as _Tr is not always an integer value, and thus each boundary has two line boundary candidates. Of these candidates, the one with the larger line number is defined as the Y element. According to these definitions, the elements of Y are variables _{k 1} and _k 2 _{(k 1} is an integer of 1 or more, _{K 2} is a non-negative integer) by using, the formula (4).

However, in practice there is an error _{T 0 (T 0 = T sym} -T fc) in the frame period _{T fc} of the symbol length _{T sym} and receiver, the error is different for each model. Therefore, the color boundary line slightly changes for each frame. Taking this into consideration, the boundary Y _real after the j-th frame from the frame in which the i-th line is adopted as the Y element is expressed by Expression (5).

Shake occurs during communication, when the screen position of the optical signal S1 varies, the use of the Y _real, time imaging time interval of the optical signal S1 is shifted from the frame period between frames, T e ₊ T _r Can be estimated within the error of

6. Symbol Error Detection FIG. 10 is a diagram illustrating a method of detecting and detecting a symbol error due to camera shake. The timing of the beginning of the frame period T _{fc and} the beginning of the symbol period T _sym are shifted. In this case, when the light receiving position is always fixed at a certain coordinate, correct data can be demodulated.

It is _assumed that the optical signal S1 is applied to the coordinate B in the preceding frame period _{Tfc_i} due to camera shake, and the optical signal S1 is applied to the coordinate A in the subsequent frame period _{Tfc_i + 1} . When the boundary line Z of the symbol is located between the coordinates A and the coordinates B in the subsequent frame period _{Tfc_i + 1} , that is, when a camera shake across the boundary line Z occurs, a symbol error occurs. Because, in this case, the preceding symbol is measured in two consecutive frames, information on the succeeding symbol cannot be obtained, and both colors of the two consecutive symbols cannot be obtained for the same coordinates. This is because the differential color shift Δ between consecutive symbols cannot be grasped. For this symbol error, a measure using an error correction code such as a Reed-Solomon code is effective. However, if the light receiving position frequently shakes across the symbol boundary line Z, a large number of symbol errors will occur, so that information cannot be correctly demodulated. In order to smoothly acquire information, it is desirable to implement a function of detecting a symbol boundary line by a receiver and notifying the user of the line on the application side.

  To detect a symbol error, it is effective to switch a set of a plurality of colors to be mapped to a signal space for each symbol period. For example, when the light source 210 can generate n colors, the receiver can detect a symbol error by dividing them into two sets and changing the set of colors used between consecutive symbols. In order to suppress flicker, it is preferable to perform grouping so that each group has the same composite color.

In the example of FIG. 10, a case is shown in which two groups of four colors are divided and 2-DCSK is performed. The odd-numbered symbols use the first set # 1e, # 2e, and the even-numbered symbols use the second set # 1o, # 2o. In the preceding frame period _{Tfc_i} , the light receiving position B is irradiated with the light signal S1 in the preceding frame period, and in the succeeding frame, the light signal S1 is irradiated in the following light receiving position A due to camera shake. You. Therefore, the receiver can recognize that the symbol boundary line Z exists between the coordinates A and B.

7. Experiment A verification experiment on DCSK communication will be described. In the experiment, the number of colors used and the communication distance were changed, and the SER during 1000 symbol transmission was measured.
7.1 Experimental Conditions As an initial evaluation, the colors to be used are two, four, and eight combinations in which the composite color is white in the CIEXYZ color space in consideration of the communication speed and the SER trade-off. Select a pattern. In each pattern, T _color is set to 6 ms, 3 ms, and 2 ms, respectively, so that the _color change is not perceived by the human eye. In the evaluation, light was diffused by using EP204K-35G1R1B1-CA as an RGB-LED and attaching a diffusion cap having a diameter of 10 mm. A one-board microcomputer Arduino UNO was used to control the LEDs. The receiver used an Asus smartphone Zenfone 2 Laser ZE601KL (Android (registered trademark) 5.0.2), and set the resolution to 1920 × 1080 and the frame rate to 30 fps. The experiment was performed indoors at an illuminance of about 500 lux, and measurements were taken by five users while holding the camera in their hands.
7.2 Experimental Results FIG. 11 is a diagram showing measurement results of SER. In DCSK using eight colors, it becomes difficult to identify colors with an increase in communication distance, and it can be confirmed that SER significantly increases. When two colors and four colors are used, the communication speed is 30 bps and 60 bps, respectively, and the maximum communication distance is 3.2 m in both cases, and the SER at this time is 0.0925, 0.104, respectively. Was. When the communication distance exceeded 3.2 m, the number of pixels by which the camera could recognize the color of the LED became 1 pixel or less, and a boundary line at which the color was switched in the screen could not be calculated, and communication was disabled.
7.3 Summary By DCSK, it can be confirmed that 60 bps communication can be realized with a maximum communication distance of 3.2 m from a small LED with a diameter of 10 mm. Since the DCSK can acquire information from the LED light that is reflected only a few pixels on the screen of the camera, the communication distance can be increased by increasing the aperture of the LED.

8. Applications Use cases of long-distance LED-camera communication include "indoor AR (Augmented Reality) browser", "lightweight implementation of indoor location estimation", and "construction of IoT device".

8.1 Indoor AR Browser FIG. 12 is a diagram illustrating an example of an AR browser. AR browsers are used for navigation in commercial facilities, translation services, and the like. A plurality of transmitters 200 are installed in the facility, and each of the transmitters 200 transmits information (here, data related to icons) related to a location where the transmitter 200 is installed to the receiver 300 using the optical signal S1. Send. The receiver 300 receives the optical signals S1 from the plurality of transmitters 200, and displays an icon (or other information) on its display so as to overlap the image of the camera.

  In an underground or indoor environment where radio waves are difficult to reach, a method that does not use GPS information is required to realize an AR service that acquires content from a range over which a smartphone camera is held. There is a marker method that allows the camera to recognize a fixed figure, but there are many operational problems in attaching and managing markers, and the appearance may be impaired. On the other hand, a method using techniques such as real space analysis and feature point extraction without using a marker has been proposed, but it is difficult to realize information transmission independent of a smartphone model from the viewpoint of the amount of calculation.

  By using long-distance LED-camera communication for these methods, intuitive information acquisition from the range over which the camera is held can be realized independent of the model.

8.2 Indoor Location Information Service There is a high demand for indoor location information services, and research and development of indoor location estimation technology is ongoing. However, high-precision and low-cost operation cannot be performed with the current indoor position estimation technology. It has been pointed out that a method using radio frequencies cannot be designed robustly against multipath reflection, shadowing and a dynamic environment. In order to solve this problem, there is a method using visible light. However, all of these methods have high human and financial costs, and the threshold for system construction is high. In the case of using an independently modulated LED, it is necessary to densely install the LED on the ceiling indoors. In the case of using fluorescent lights already installed in the building, it is not necessary to install new lighting equipment, but it is necessary to measure the characteristic amount of the light emitted from the fluorescent lights in advance, and perform this for each smartphone model Is a high threshold.

  By using the long-distance LED-camera communication for these methods, the number of LEDs to be installed can be significantly reduced, and indoor position estimation can be realized at low cost. Specifically, LEDs are installed at intervals of several tens of meters and each transmits a unique ID (identifier). The camera of the smartphone can estimate the current indoor position by using the received ID and information on the distance and angle to the light source.

8.3 Construction of IoT Device The construction of an IoT device requires a function of visualizing sensor data such as temperature and illuminance. By using the long-distance communication described in the present embodiment, it is possible to specify an IoT device that transmits information and to transmit a small amount of data such as a node ID.

8.4 Conclusion By adopting DCSK, it is possible to extend the communication distance to several tens of meters or more by adjusting the size and power consumption of the LED. Therefore, an intuitive information acquisition application such as the indoor AR browser shown in FIG. 12 can be realized at low cost and versatile. In the present communication system, information can be transmitted only by adjusting the color switching pattern of the LEDs, so that introduction and operation of the communication system can be suppressed at low cost. In addition, since the LED as the transmitter can be used for lighting purposes, it can be operated without deteriorating the beauty of the use environment.

  On the other hand, in a system using Bluetooth (registered trademark) or Wi-Fi, it is necessary to select a communication target from a plurality of devices located around several meters around, and it is difficult to realize communication intuition. In the method using NFC, applications are limited because the communication distance is short. Among existing AR technologies, there is a method that can acquire contents from space even in indoor environments where it is difficult to use GPS functions by using technologies such as real space analysis, but it is independent of smartphone models It is difficult to realize information transmission. In contrast to these methods, the long-distance LED-camera communication method can easily identify a plurality of communication target devices from a range over which the camera is held and acquire information. In addition, the present invention is not limited to the latest smartphone having a high calculation function, and can realize information transmission independent of a model.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and it is understood by those skilled in the art that various modifications can be made to the combination of each component and each processing process, and that such modifications are also within the scope of the present invention. is there. Hereinafter, such modifications will be described.

<Modification>
In the modified example, as in DCSK, the color of the optical signal S1 repeats # 1 to #n for one symbol period. Then, the leading color of each symbol period is determined based on only the transmission data without depending on the previous symbol. For example, when the signal space of FIG. 3B is used, for the transmission data “00”, “01”, “10”, “11”, the leading colors of the symbols are set to # 1, # 2, # 3, # 4. For example, in the symbol period corresponding to the transmission data "01", the color of the optical signal S1 circulates # 2, # 3, # 4, # 1,.

This modification is effective when the transmitter 200 and the receiver 300 perform one-to-one communication, and the timing of the transmitter 200 and the receiver 300 can be adjusted prior to the start of communication. In this case, data transmission becomes possible by aligning the head timing of the symbol period of the transmitter 200 with the head timing of the frame period _Tfc in the receiver 300.

  According to this modification, it is not necessary to irradiate light over all the pixels of the camera, and communication using a small optical signal S1 that irradiates only a few pixels with spots is possible as in DCSK. , Telecommunications are possible. Also, similarly to DCSK, data transmission is possible even if the light receiving position of the optical signal S1 changes.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments are merely illustrative of the principles and applications of the present invention, and the embodiments are defined in the appended claims. Many modifications and changes in arrangement may be made without departing from the spirit of the present invention.

Reference Signs List 100 optical communication system 200 transmitter 210 light source 220 modulator 300 receiver 310 camera 320 demodulator S1 optical signal S2 transmission data S3 reception data

Claims (13)

  During the symbol period, the color of the optical signal is cyclically changed by a plurality of predetermined colors, and a head color of each symbol period is selected from the plurality of colors based on transmission data. Modulation method.   The light modulation method according to claim 1, wherein the leading color of each symbol period is relatively determined based on the leading color of the immediately preceding symbol period.   The optical modulation method according to claim 1, wherein a length of the symbol period is equal to a frame rate of a camera that captures the optical signal.   4. The light modulation method according to claim 1, wherein the set of the plurality of colors is switched for each symbol period. An optical demodulation method of an optical signal modulated by the optical modulation method according to any one of claims 1 to 4,
Capturing the light signal with a rolling shutter camera;
Identifying a pixel containing the optical signal;
Obtaining received data based on the color obtained at the pixel;
An optical demodulation method comprising: The leading color of each symbol period is relatively determined based on the leading color of the previous symbol period,
6. The optical demodulation method according to claim 5, wherein the received data is obtained based on a relationship between a color obtained in each frame and a color obtained in a previous frame in the pixel. .   When pixels including the optical signal are different in the scanning direction of the camera in two consecutive frames, the method further includes a step of shifting a color acquired at one pixel and estimating a color to be detected at the other pixel. The optical demodulation method according to claim 5, further comprising: A light source capable of generating a plurality of predetermined colors;
A modulator that controls the color of an optical signal generated by the light source based on transmission data,
With
The modulator, wherein, during a symbol period, the color of the optical signal is cyclically shifted by the plurality of colors, and a leading color of the symbol period is changed based on the transmission data. Machine.   The transmitter according to claim 8, wherein the leading color of each symbol period is relatively determined based on the leading color of the immediately preceding symbol period.   The transmitter according to claim 8, wherein a length of the symbol period is equal to a frame rate of a camera that captures the optical signal.   The transmitter according to any one of claims 8 to 10, wherein the set of the plurality of colors is switched for each symbol period. A camera of a rolling shutter system for capturing an optical signal from the transmitter according to claim 9,
A demodulator that processes the output of the camera;
With
The demodulator identifies a pixel containing the optical signal and obtains received data at the pixel based on a relationship between a color obtained in a current frame and a color obtained in a previous frame. Receiver.   When pixels including the optical signal are different in the scanning direction of the camera in two consecutive frames, the method further includes a step of shifting a color acquired at one pixel and estimating a color to be detected at the other pixel. The receiver according to claim 12, comprising:

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